In recent years, there has been vigorous development of high-breakdown voltage, high-output compound semiconductor devices, making use of advantages of nitride-based compound semiconductor including high saturation electron mobility and wide band gap. The development is directed to field effect transistors such as high electron mobility transistors (HEMTs), for example. Among them, a GaN-based HEMT having an AlGaN layer as an electron supply layer attracts a lot of attention. In the GaN-based HEMT, lattice distortion occurs in the AlGaN layer due to difference in lattice constants between AlGaN and GaN, the distortion induces piezo polarization therealong, and thereby generates a high-density, two-dimensional electron gas, in the upper portion of the GaN layer laid under the AlGaN layer. This configuration ensures high output.
However, it is very difficult to manufacture a GaN substrate with a good crystallinity. Major conventional solutions have been such as forming a GaN layer, AlGaN layer and so forth by hetero-epitaxial growth, over a Si substrate, sapphire substrate, SiC substrate or the like. In particular as for Si substrate, those having large diameter and high quality are readily available at low costs. Investigations into structures, having a GaN layer and an AlGaN layer formed over the Si substrate, have therefore been flourishing.
Growth of the GaN layer directly on the Si substrate, however, allows Ga and Si to react with each other in the process of growth. Investigations have, therefore, been directed to techniques of forming an AlN layer as a buffer layer, in advance of the growth of the GaN layer.
Formation of the AlN layer directly over the Si substrate, however, degrades the breakdown voltage, due to carriers possibly generated in the vicinity of the interface between the both. In addition, formation of the AlN layer is still insufficient for fully suppressing the reaction between Ga and Si.
[Patent Literature 1] Japanese Laid-Open Patent Publication No. 11-274082
[Patent Literature 2] Japanese Laid-Open Patent Publication No. 2002-110569
[Non-Patent Literature 1] H. Umeda, et al., IEDM Technical digest 2010, pp. 482